Do you get more bang for the buck if you run 5 gallons E85 with a fill instead of 3 gallons?

You are only using the Ethanol to clean up timing corrections & run aggressive tunes, and a bit more ponies. it you are not properly tuned you really are not getting any power benefit per say, from 5 gallons.

Ethanol is less dense, thus it takes more fuel than regular pump gas. I haven't done any comparison, but, I am getting about 15 mpg with 50% Ethanol vs my 18 mpg I was getting with ACN 91. As my DD

And of course the E50 Tune is for performance not for MPGs.

if you wanted to do the math, 8.5 gallons of E85 and 7.5 Gallons of 91 is my current setup.

In case you're interested these were my results from playing with E85 on stock hardware. Results not corrected for elevation. My custom E85 map is fairly similar to the E30 OTS map but fuel is adjusted for a higher E85 content. I found the E30 map to be a bit too aggressive in timing for my tastes on only 30ish % E85.